The present invention relates to photosensitive materials for aligning liquid crystals, liquid crystal displays, and other liquid crystal optical elements.
Common to almost all liquid crystal based devices is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fiberous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. Nos. 5,032,009 and 4,974,941 “Process of Aligning and Realigning Liquid Crystal Media”.
The process for aligning liquid crystal media with polarized light can be a noncontact method of alignment that has the potential to reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios, long-term thermal and optical stability and in many applications a controlled uniform pre-tilt angle. Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer aligns parallel to the buffing direction, but is not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2–15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
In a continuing effort to develop optical alignment materials for commercial applications U.S. Pat. No. 6,103,322 (Gibbons et al.) describes novel polyamic acids comprising a dianhydride containing a diaryl ketone group, i.e. two aromatic rings linked by a carbonyl group, (e.g. benzophenonetetracarboxylic dianhydride) and an aromatic diamine containing an unsaturated alkyl side chain (e.g. geranyl or allyl group). Gibbons et al, recognized that polyamic acids combining these two structural units gave good quality alignment and better electrical properties (specifically, voltage holding ratio, VHR) than similar materials that lacked the unsaturated alkyl side-chain.
Herein are described new reactive side-chain polymers within the class of polyimides, polyamic acids and esters thereof, that give good quality alignment of liquid crystals and generate VHR properties superior to those previously achieved with optical alignment processing.